1. Field of the Invention
The present invention relates to the communication systems using optical signals propagating through single-mode optical fibres and, in particular, a method of and an apparatus for generating, transmitting and receiving a multilevel optical signal.
2. Description of the Related Art
Reliable and economically competitive, coherent optical transmission systems which can be made available at short and medium terms allow novel network architectures to be provided regarding long-distance and high-performance connections and multi-user LAN (Local Area Network) and MAN (Metropolitan Area Network) connections as well. In particular, the very large bandwidth of the single-mode optical fibres (thousands of GHz) can be suitably exploited by providing optical FDM-systems (Frequency Division Multiplexing) in which the selection of the desired channel can be obtained by shifting the frequency of the local oscillator. This allows passive optical networks with very high traffic capacity (thousands of gB/s) to be carried out. However, two important aspects restrict on one hand the bandwidth of the single channel and limit on the other hand the maximum number of channels which can be tuned by the user. In the first instance, in fact, the main restriction is due to the bandwidth of the photodiodes and the electronic circuits, while regarding the second instance it should be considered that the frequency range which can be tuned by the user depends on the tunability characteristics of the laser used as local oscillator.
In order to increase the information rate of any channel, systems have been provided in which the information to be transmitted is coded with more than two levels instead of being coded using only the two binary levels as it is customary for providing a high signal reception sensitivity. By transmitting multilevel signals an improvement of spectrum efficiency expressed in terms of information rate per unit of occupied band is obtained at the cost of a reduction of the sensitivity. The known systems with two or more levels resort to the digital amplitude and phase keying (APK) or to the digital phase shift keying (PSK) or polarization shift keying (SPSK) of the electrical component of the electromagnetic field associated to a coherent optical wave generated by a laser source.
In particular, according to the previous state of art, EP-A-0 277 427 discloses methods of an devices for processing an optical signal by altering the polarization state thereof under control of a signal at a predetermined scrambling frequency.
EP-A-0 280 075 discloses an optical low-noise superheterodyne receiver for modulated optical signals in which a received light signal is coupled to a coherent light signal having the same polarization. Then such signals are combined so as to provide two pairs of optical signals, the signal of each pair having the same polarization perpendicular to that of the other pair, and fed to photoelements which provide electrical signals. Such electrical signals are then summed to each other after demodulation and after at least a phase shifting of one of such signals.
In "Electronics Letters" Vol. 26, No. 4 of 15 Feb. 1990 there is disclosed the performance of coherent optical transmission systems using multilevel polarization modulation based upon equipower signal constellations at the vertices of regular polyhedra inscribed in to the Poincare's sphere.